1. Field of the Invention
The present invention relates to a temperature compensated oscillator, an adjusting method thereof, and an integrated circuit for temperature compensated oscillator. The invention more particularly relates to a temperature compensated oscillator that corrects frequency fluctuations caused by changes in ambient temperatures by digital control, an adjusting method thereof, and an integrated circuit for temperature compensated oscillator.
2. Description of the Prior Art
A conventional system having a timer function such as a clock and an RTC (Real Time Clock) normally uses a crystal oscillator with a tuning fork crystal resonator operating at 32.768 KHz or an AT cut crystal resonator operating at 4.194304 MHz.
The tuning fork crystal resonator operating at 32.768 KHz can oscillate with low current consumption but suffers from disadvantageous frequency fluctuations caused by temperature changes. Meanwhile, heat from the user""s body can change the temperature of the outer part of a watch, but the surrounding part of the resonator is kept at a relatively fixed temperature, and therefore the low current consumption characteristic of the tuning fork resonator can be advantageous in this application.
Conversely, the AT cut crystal resonator operating at 4.194304 MHz suffers less from frequency fluctuations caused by temperature changes than the tuning fork type, while it requires large current for oscillation.
Conventionally, the advantages and disadvantages are balanced against each other, and either the tuning fork type or the AT cut type has been used. In recent years, however, with increasing demands for lower power consumption systems, the AT cut type resonator has been less frequently used. As a result, the disadvantage of the tuning fork type resonator in association with the temperature characteristic has been more often pointed out.
The frequency-temperature characteristic of the tuning fork type crystal resonator can generally be approximated by the following quadric:
xcex94f/f=A2(Txe2x88x92T0)2+A0 
where T0 represents the reference temperature that as well as the coefficient of the quadric varies among crystals. FIG. 16 shows an example of a frequency-temperature characteristic.
Meanwhile, the oscillation frequency of a crystal oscillation circuit is as follows:
f0=fs(1+1/(2C0/C1(1+CL/C0)) 
where fs, C0, and C1 represent the resonance frequency, equivalent parallel capacitance, and equivalent series capacitance of the crystal resonator, respectively. CL represents the load capacitance of the oscillation circuit. From the equation, assuming that load capacitance CL is variable with temperature T, the frequency can be adjusted, so that temperature compensation can be carried out. An example of the variable characteristic of the frequency depending upon load capacitance CL is shown in FIG. 17.
As can be understood from FIG. 16, in the temperature range from xe2x88x9235xc2x0 C. to 85xc2x0 C., a frequency deviation in the range from xe2x88x9220 ppm to 200 ppm at most must be compensated. Meanwhile, from FIG. 17, around the standard value of load capacitance CL, 6 pF, load capacitance CL must be controlled to be in the range from 1.9 pF to 7.2 pF to achieve the above compensation. In practice, the parasitic capacitance of the input/output portion of the IC is about 1 pF (at the pad or the protection circuit), and therefore as only for the variable capacitance element, the capacitance must be controlled between 0.9 pF to 6.2 pF. At present, it is substantially impossible to control the capacitance ratio of 6.2 pF/0.9 pF=6.89 using a variable capacitor stored in the IC, and with low power supply voltage.
What is generally practiced at present is as follows. The load capacitance CL of an oscillation circuit 181 shown in FIG. 18 is made of a capacitor array 182 as shown in FIG. 19. Switching elements SW0 to SWnxe2x88x921 are turned on/off to selectively connect capacitive elements C0 to Cn-1 to the oscillation circuit 181 and control load capacitance CL. An analog signal output from a temperature detector 183 is converted into a digital signal by an A/D converter 184, compensation data is read out from a memory 185 using the digital signal as an address, and the switching elements SW0 to SWn-1 are turned on/off based on the compensation data.
Compensation is carried out for each predetermined temperature step, and therefore errors on both limits of the temperature range increase when the temperature range expands. In order to reduce the errors, the temperature step width must be reduced, which increases the bit number of the memory 185. This is illustrated in FIGS. 20 and 21. The minimum control unit of the capacitor array 182 in FIG. 19 must also be reduced, which also increases the bit number of the memory 185.
According to the conventional method, the load capacitance (capacitors) CL of the oscillation circuit is arranged in an array form and controlled by turning on/off the switching elements. According to this method, in order to improve the adjusting precision, the area size of the capacitor array and the bit number of memory are inevitably increased. This makes it difficult to reduce the cost.
It therefore an object of the present invention to provide a temperature compensated oscillator keeping the area of the capacitor array and the bit number of the memory from increasing and allowing for high precision and an adjusting method thereof.
Preferably, a temperature compensated oscillator according to the present invention includes a temperature detector that outputs an analog signal depending on a temperature, an A/D converter for converting the analog signal from the temperature detector into a digital signal, a memory from which compensation data is read out using the digital signal from the A/D converter as an address, a capacitor array for selectively connecting a plurality of capacitive elements to an oscillation circuit based on the compensation data, the oscillation circuit causing a resonator such as a crystal resonator to oscillate thereby generating an oscillation output signal, and using the capacitor array as a frequency adjusting element for the oscillation output signal, a frequency comparison circuit for comparing the frequencies of an externally input reference frequency signal and the oscillation output signal, a register in which the value of each bit is sequentially determined based on the frequency comparison result from the frequency comparison circuit, a switching circuit for selectively supplying the compensation data read out from the memory and a digital signal output from the register to the capacitor array, a voltage variable capacitive element connected to the capacitor array, and a control voltage generation circuit for generating control voltage to control the capacitance of the voltage variable capacitive element in response to the analog signal from the temperature detector. The digital signal output from the register is supplied to the capacitor array through the switching circuit and the plurality of capacitive elements are connected to the oscillation circuit in response to the digital signal, so that the oscillation circuit carries out oscillation operation. The value of each bit in the register is sequentially determined based on the comparison result for each comparison operation by the frequency comparison circuit to change the frequency of the oscillation output signal. The digital signal output from the register when the frequency of the oscillation output signal from the oscillation circuit is matched with a particular frequency is written in the memory as the compensation data corresponding to the detection temperature that can be addressed using the digital signal output from the A/D converter corresponding to the temperature detected by the temperature detector at the time, so that the writing operation is carried out for each temperature step.
Preferably, in the above temperature compensated oscillator, a point where a prescribed frequency difference is generated between the oscillation output signal and the external reference frequency signal is regarded as a breakpoint between the temperature steps.
Preferably in the above temperature compensated oscillator, in a first process a point where a prescribed frequency difference is generated between the oscillation output signal and the external reference frequency signal is regarded as a breakpoint between the temperature steps, and the frequency of the oscillation output signal is matched to the frequency of the external reference frequency signal, while in a second process a point where there is substantial coincidence between the frequency of the oscillation output signal and the frequency of the external reference frequency signal is regarded as a breakpoint between the temperature steps, and the frequency of the oscillation output signal is matched to a frequency shifted from the external reference frequency signal by the prescribed frequency difference, and the first and second processes are switched at a particular temperature. Also preferably, in this temperature compensated oscillator, the frequency of the external reference frequency signal is set to a frequency shifted from a desired frequency by half of the prescribed frequency difference. Here, also preferably, the frequency of the external reference frequency signal is prevented from being shifted in the vicinity of the particular temperature. Furthermore, the resonator is preferably a tuning fork type crystal resonator and the particular temperature is preferably a peak temperature in the frequency-temperature characteristic of the resonator.
Preferably, a temperature compensated oscillator according to the present invention includes a capacitor array having a plurality of selectively connected capacitive elements forming a variable capacitance, a voltage variable capacitive element connected to the capacitor array and forming a load capacitance with the capacitor array, a temperature detector that generates an output voltage depending on a temperature, a control voltage generation circuit for generating control voltage for temperature compensation to control the voltage variable capacitive element based on the output voltage of the temperature detector, a memory storing compensation data for each of a plurality of temperature steps, and a control circuit responsive to the output voltage of the temperature detector for reading out the compensation data corresponding to the temperature from the memory and connecting a corresponding one of the capacitive elements of the capacitor array based on the compensation data. Also preferably, a resonator assumed to have a frequency-temperature characteristic symmetric with respect to a peak temperature in a temperature range for temperature compensation is used, and the control voltage generation circuit generates control voltage having a temperature characteristic symmetric with respect to the peak temperature in the frequency-temperature characteristic of the crystal resonator.
Preferably, by a method of adjusting a temperature compensated oscillator according to the present invention, the temperature compensated oscillator includes a temperature detector that outputs an analog signal depending on a temperature, an A/D converter for converting the analog signal from the temperature detector into a digital signal, a memory from which compensation data is read out using the digital signal from the A/D converter as an address, a capacitor array for selectively connecting a plurality of capacitive elements to an oscillation circuit based on the compensation data, the oscillation circuit causing a resonator such as a crystal resonator to oscillate thereby generating an oscillation output signal, and using the capacitor array as a frequency adjusting element for the oscillation output signal, a frequency comparison circuit for comparing the frequencies of an externally input reference frequency signal and the oscillation output signal, a register in which the value of each bit is sequentially determined based on the frequency comparison result from the frequency comparison circuit, and a switching circuit for selectively supplying the compensation data read out from the memory and a digital signal output from the register to the capacitor array. The digital signal output from the register is supplied to the capacitor array through the switching circuit and the plurality of capacitive elements are connected to the oscillation circuit in response to the digital signal, so that the oscillation circuit carries out oscillation operation. The value of each bit in the register is sequentially determined based on the comparison result for each comparison operation by the frequency comparison circuit to change the frequency of the oscillation output signal, the digital signal output from the register when the frequency of the oscillation output signal from the oscillation circuit is matched with a particular frequency is written in the memory as the compensation data corresponding to the detection temperature that can be addressed using the digital signal output from the A/D converter corresponding to the temperature detected by the temperature detector at the time, so that the writing operation is carried out for each temperature step.
Preferably in the above adjusting method, a point where a prescribed frequency difference is generated between the oscillation output signal and the external reference frequency signal is regarded as a breakpoint between the temperature steps.
Preferably in the above adjusting method, in a first process a point where a prescribed frequency difference is generated between the oscillation output signal and the external reference frequency signal is regarded as a breakpoint between the temperature steps, and the frequency of the oscillation output signal is matched to the frequency of the external reference frequency signal, while in a second process a point where there is substantial coincidence between the frequency of the oscillation output signal and the frequency of the external reference frequency signal is regarded as a breakpoint between the temperature steps, and the frequency of the oscillation output signal is matched to a frequency shifted from the external reference frequency signal by the prescribed frequency difference, and the first and second processes are switched at a particular temperature. Also preferably in the above adjusting method, the frequency of the external reference frequency signal is set to a frequency shifted from a desired frequency by half of the prescribed frequency difference. Also preferably in the above adjusting method, the frequency of the external reference frequency signal is prevented from being shifted in the vicinity of the particular temperature. Furthermore, the particular temperature is preferably a peak temperature in the frequency-temperature characteristic of the resonator.
Preferably, an integrated circuit for temperature compensated oscillator corresponding to the above temperature compensated oscillator and the adjusting method thereof is provided.